In many environments it is required to transfer fluid between bodies in relative motion to one another. For example, in a marine environment, fluids such as hydrocarbon fuels are often loaded onto ships. In such an environment the floating ship operates with a degree of movement which must be accommodated.
These issues occur when a ship is supplied from the shore and also when ships are supplied by other floating vessels. For example, a bunker vessel may be used to refuel a larger ship. In recent years, the use of liquid natural gas (LNG) as a fuel for maritime vessels has increased in popularity meaning that the provision of a bunker vessel able to refuel an LNG-fuelled ship is increasingly desirable, in addition to the need to transfer LNG between vessels.
Many prior art transfer techniques utilise articulated arms comprising a plurality of rigid sections pivotally mounted to one another to allow for degrees of movement. While this approach has resulted in reliable and robust transfer arms, there remain difficulties with the practical implementation of such systems in a range of circumstances.
For example, there are substantial challenges in developing transfer arms comprising rigid members which have a sufficient range of movement. The range of movement required increases in particular when the transfer occurs between two floating vessels. Moreover, to provide appropriate degrees of movement at each joint in the articulated arm, the pipe through which the fluid is transferred is often disposed at a central axial point. This means that in many designs the arm can only carry a single pipe, or that incorporating an additional line results in significantly increased complexity. This is a particular disadvantage in the transfer of liquid natural gas, since it is often necessary not only to transfer LNG to the receiving vessel but to carry boil off gas (i.e. LNG vapour) away from that vessel. There are difficulties in providing the vapour line in rigid arm solutions.
In addition to the above, transfer apparatuses of the type described above can be difficult to maintain and awkward to manoeuvre. In particular, rigid arm solutions are typically heavy with each arm generally balanced around their pivot by counter weight. The outcome of this arrangement is that large forces can be generated at the receiving vessel manifold when the relative motion increases in amplitude or speed.
In response to the difficulties outlined above, solutions have been proposed which use flexible hoses to transfer the fluid. The flexibility of the hoses is able to accommodate some of the relative movement that occurs due to the floating vessel or vessels. However, it is not appropriate merely to allow hoses to be extended between the receiving vessel and the source in an uncontrolled manner. Control is required when managing the connection and disconnection of the apparatus, but is also beneficial during any fluid transfer. One particular requirement when delivering fuel is that it is necessary to have a mechanism to handle emergency disconnection in an unexpected situation. For example, it is necessary to provide a mechanism to move the hose to a safe location should such a disconnection occur.
One proposed solution is described in International Patent Application No. WO2013/064601. This document describes a fluid transfer hose manipulator in which the hose is supported by an articulated arm which both maintains the hose in position and maintains it under tension. In this way, a structure is provided for the flexible hose. The structure comprises a plurality of hose guides at each of the connecting points in the articulated arm to ensure that the hose follows the structure of the arm. The articulated arm also comprises a hose tensioner which ensures that the hose is maintained under tension at all times.
While this approach provides some flexibility, it is dependent upon the reliability of the hose. The hose is under tension and is repeatedly flexed as relative motion occurs It is therefore necessary to ensure that any hose used is capable of withstanding such usage and to provide appropriate maintenance, perhaps repeatedly replacing the hose over the lifetime of the apparatus. This increases the complexity and expense of the solution. The issues involved are particular complex when the fluid carried is LNG, since this is carried at low temperatures (typically around −163° C.) and thus placing its own limits on hose materials and design.
There is therefore a continuing desire to provide a transfer apparatus which provides advantages in both reliability and flexibility. At the same time, it is important that any apparatus complies with the stringent safety requirements for the transfer of flammable fluids, particularly at sea, by providing suitable control.